Intro music
Hello! It's Scott Manley here.
Now, I like to talk about the educational merits of Kerbal Space Program.
But I am acutely aware there are many things that it doesn't in fact cover in the field of rocket science...
such as nozzles.
Now nozzles are what many people think of as "rockets".
They are the bell shaped business end of the rocket...
which expel firey awesomeness during the launch process.
But in reality it is just one part of a whole rocket engine.
In the rocket engine what you have is fuel and oxidizer being mixed inside a combustion chamber.
And then these exhaust gases escape out through a nozzle.
Which is carefully designed by rocket scientists...
to harness the most thrust and efficiency..
from those chemical reactions.
The classic nozzle design actually predates rocket science.
The "De Laval" nozzle was originally designed for a steam turbine in the nineteenth century.
And what happened in the "De Laval" nozzle is:
you have high pressure exhaust gases being forced through a small throat.
And this is narrow enough that the gases end up getting choked at the speed of sound.
Beyond this the nozzle expands outwards
and because of the magic of fluid dynamics
as the pressure drops and they travel down
the gasses accelerate and they end up going at supersonic speeds.
and of course in rockets, the faster those exhaust gasses go
the more efficient your rocket is.
Getting the right nozzle shape and dimension
can make a huge difference to the performance of a rocket engine.
and uh, the atmosphere complicates these things
you see, it turns out that the ideal exhaust gas pressure is the same as the external atmospheric pressure
Now this means that for booster rockets designed to be used at sea level, their engine bells will be smaller,
whereas engines designed to be used in the vacuum of space will have much longer and wider nozzles
to try and get the gas pressure, or the exhaust gas pressure down close to that of the vacuum.
the Merlin engines on the Falcon 9 are a perfect example of this.
Since the first and second stage engines are almost identical,
but on the first stage there is room for 9 engines and their associated nozzles
but on the second stage there is a single engine designed for vacuum
and its engine bell is almost as wide as the rocket itself.
As large as it is, this second stage engine bell is a compromise.
To actually get the exhaust gas pressure down to match the vacuum of space would require an infinitely large nozzle.
So engineers pick a compromise size which gets the gas pressure down low enough
while still fitting inside the second stage fairing.
incidentally there is funny story about that second stage engine nozzle.
During an early launch they discovered cracks on the engine bell extension and it was decided to fly out
an engineer who would enter into the interstage and with a pair of metal snips
he essentially snipped off the bottom four feet of the rocket nozzle and the
rocket flew and they didn't need the extra performance that the engine bell gave.
This is of course in our actual bona-fide real life example of rocket surgery!
The designers of the RL 10 rocket which is used in the center
have a neat trick for actually fitting their engine nozzle into an interstage
some versions of it have a nozzle extension which comes down and
and extends the nozzle after the staging happens so this way they can make the interstage
smaller and save on size if not complexity
back inside the atmosphere things are a little more complicated
The ideal nozzle matches the exhaust pressure to the atmospheric pressure
but Rockets are generally known for going up
and as they go upwards they will pass through different air pressure so
the perfect design for sea level is not as good once you reach higher altitudes
and ultimately space
typically the range and nozzles for a launch vehicle booster are slightly over-expanded
at launch and they'll reach some perfect performance at altitude and then
as they blaze skywards it will become under expanded by the time the engine cuts out
but there's actually a limit to how big you can make a simple rocket nozzle
and still have the engine operate stably. If the exhaust pressure
gets too low then the atmosphere can push back inside the rocket nozzle
and this causes the exhaust to detach from the rocket
and the engine nozzle walls and this can cause all sorts of instability
that can lead to control problems and other undesirable phenomena
typically this occurs at about forty percent of atmospheric pressure for a
simple nozzle. The Space Shuttle main engine is an interesting example in this
regard, if you recall the space shuttle would light its main engines on the
ground and use them all the way into space
so if they need to they needed to design an ancient which would work well at all altitudes
The designed exhaust pressure is about thirteen percent of atmospheric pressure
that's below the forty percent that of course would lead to flow separation and instability
So the designers redesigned their nozzle very slightly that towards the end as it
curves outwards they suddenly change the curvature in a little
and that surprises the exhaust gases just enough the pressure rises near the
edge and keeps its pinned to that surface and keeps the combustion stable
this provides decent performance on the ground and all the way up into space
although they weren't as good as they could have been because they did originally
have a proposal for a longer nozzle extension and another thing over
expanded nozzles are the source of those beautiful chains of shock diamonds in exhausts
Because the exhaust gases for an over expanded nozzle are below that of
atmospheric pressure as they come out they get squeezed in and then the
pressure gets too high and the bounce back so you get a hot spot as it bounces
in and out and that's why you'll get the chain of shock diamonds on certain types
of engines, of course as the engines go higher up in the atmosphere the pressure
drops and the nozzle becomes no longer over expanded so it's a phenomena that
is related to exhaust gas pressure versus atmospheric pressure you don't
see it in space incidentally now I focused on conventional nozzles
but because of the altitude dependence there is an incentive for scientists
come up with the nozzle designs are efficient and across a wider range of
atmospheric pressures and there's been a number of different altitude
compensating designs the Kerbal space program players are probably most
familiar with the toroidal aerospike engine which has a ring-shaped
combustion chamber that pushes the gases inwards towards a spike shape structure
and this provides a performance which is more consistent across different
atmospheric pressures but it's not the only altitude compensating design
there are other things like plug nozzles linear aerospikes and of course
engine bells that drop down like on the RL 10 Anyway, nozzle geometry is a
critically important part of rocket design and oddly enough it's one of
those places that kerbal space program frequently gets it wrong
since the engine designs were built by modelers who weren't really rocket scientists
for example the 'Poodle' engine-bell is a lot smaller than say the 'Mainsail'
and yet gets much better efficiency (better specific impulse)
I hear one day this may get fixed, regardless I hope to be back with more
rocket science that kerbal space program doesn't teach you
I'm Scott Manley, fly safe.